This invention relates generally to an internal combustion engine and particularly to an improved carburetor for optimum control of an air-fuel mixture supply to the engine.
It is widely recognized that abnormally high emission of hydrocarbons occur in the engine exhaust under deceleration where the engine is acting as a brake and the throttle valve is fully closed. There are several interdependant reasons for this: Owing to a high vacuum in the intake manifold, a considerable amount of exhaust gas is drawn back into the cylinders so that the combustible mixture is too lean for normal ignition. Besides, the quantity of mixture fed into each cylinder is reduced to a minimum, which again results in failure in firing of the mixture. Thus the products of incomplete combustion containing a high concentration of free hydrocarbons are discharged with the exhaust gases into the atmosphere.
In order to meet the severe statutory requirements for emission control, there is an increasing tendency to equip internal combustion engine driven vehicles with catalytic converters for treating engine exhausts. The unburned hydrocarbons admitted into the catalytic converter produce reaction heat upon oxidization therein. Due to the heat, the operating temperature of the catalyst rises to an abnormal level, with the result that the activity of the catalyst falls and often the catalyst itself is damaged. Accordingly, minimization of the production of unburned hydrocarbons prior to the engine exhaust cleaning system is required from the standpoint of emission control.
There have been proposed a number of solutions to this problem, one of which is to arrange for an additional or supplementary mixture to be admitted downstream of the throttle valve for reduction of vacuum and for complete combustion of fuel. One exemplary arrangement of this is a by-pass passage for additional mixture provided within the carburetor, the passage being opened by a valve sensitive to high vacuum in the intake manifold to allow additional mixture into the intake manifold. There is another example wherein the throttle valve normally substantially closed upon engine deceleration is mechanically moved to a position of greater opening when the manifold vacuum exceeds a predetermined level.
These arrangements are, however, accompanied by some drawbacks: the supply of additional mixture reduces the braking effect of the engine and therefore badly affects the riding qualities when the vehicle is running down a hill. Also, it is undesirable in terms of fuel economy to consume fuel throughout deceleration. If the quantity of additional mixture is so limited as to maintain the manifold vacuum at a certain level for the purpose of reducing fuel consumption, not only is the quantity of mixture insufficient for full combustion but sufficient compression pressure for firing cannot build up when the manifold vacuum exceeds that level. This results in misfiring and therefore emission of hydrocarbons.
There is another proposal directed to engine exhaust control during deceleration in which, in contrast to the teaching of supplying additional mixture as described above, the mixture supply through any passageway including idle or slow port is completely cut off as long as the engine is driven by the vehicle with the throttle valve closed, thereby eliminating emission of unburned hydrocarbons. While fuel consumption may be minimized by this expedient, this has been proved impractical because, after the engine speed is decreased to appropriately idling speed in the progress of deceleration, or when the deceleration occurs at a relatively low vehicle speed as is rather usual when the vehicle is running on an urban street, the engine tends to stall due to the lack of combustible mixture. Thus, the fuel deposited on the walls of the intake system is discharged in a form of toxic unburned gas into the atmosphere.